Process for argon and nitrogen production

ABSTRACT

A process comprising: subjecting a process gas containing NOx to a stage for absorption of NOx in a suitable absorption means, obtaining nitric acid and a tail gas containing nitrogen, argon and residual NOx; subjecting said tail gas to a treatment which comprises at least one NOx removal stage, obtaining a conditioned tail gas; subjecting at least a portion of said conditioned tail gas to a separation treatment, obtaining a product stream containing argon and a product stream containing nitrogen.

FIELD OF APPLICATION

The invention relates to the technical field of argon and nitrogenproduction.

PRIOR ART

Argon (Ar) is a noble gas and as such is chemically inert. Thanks tothis property, it finds numerous industrial applications, for example inthe formation of inert atmospheres.

Thanks to its low reactivity, molecular nitrogen (N₂) is alsoparticularly suitable for the creation of inert atmospheres in differentindustrial and technological environments. Large quantities of nitrogenare also used for cryogenic applications, but the main use remains thesynthesis of ammonia, from which fertilizers, polymers, explosives andcolorants are obtained.

Argon and nitrogen are commonly obtained from an air fractionationprocess, together with oxygen.

Most air fractionation plants operate a fractional distillation processof liquid air. Said process is known as cryogenic process andsubstantially separates argon, nitrogen and oxygen, making use of theirdifferent boiling points, which are respectively −186° C., −196° C. and−183° C. Air liquefaction processes known in the art are, for example,the Linde process and the Claude process. Fractional distillation isgenerally performed in a system comprising several distillation columns,typically three columns.

One problem of this technology is that argon and oxygen have boilingpoints which are very close to each other, and this makes it difficultto obtain high purity argon, separating it from the oxygen. Nitrogenobtained with this technology also generally contains ppm levels of Arand oxygen, which are undesirable.

In order to promote the separation of argon and nitrogen as much aspossible, large-size distillation columns with a large number of platesmay be used, the number of columns may be increased, or adsorption bedsmay be installed downstream of the columns thus performing a furtherpurification of argon and nitrogen streams obtained from distillation.However, all these solutions are costly in terms of plant design andenergy consumption.

Other plants which produce argon and nitrogen use a selective adsorptionprocess through membranes. However, these plants are still not verywidespread and are rather costly.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a process which allowsto obtain substantially pure argon and nitrogen and which is at the sametime both simple and low-cost.

The applicant has found that the tail gas of a process for the synthesisof nitric acid, in view of its composition, is a convenient source forproducing argon and nitrogen.

As is known, the synthesis of nitric acid according to the Ostwaldprocess comprises an absorption step of nitrogen oxides NOx in water,which produces a stream of nitric acid and a tail gas containingnitrogen, argon, residual NOx and optionally also N₂O. In the prior art,said tail gas is generally treated to remove NOx and possibly N₂Oaccording to the prescribed limits for emissions into atmosphere, andthen discharged. The idea underlying the invention is that of treatingat least part of said tail gas in order to separate the argon andnitrogen contained therein, thus obtaining products with a highcommercial value.

The aforementioned objects are achieved with a process according toclaim 1, which comprises the following steps:

subjecting a process gas containing NOx to a NOx absorption stage in asuitable absorption means, obtaining nitric acid and a tail gascontaining nitrogen, argon and residual NOx;

subjecting said tail gas to a treatment comprising at least one NOxremoval stage, obtaining a conditioned tail gas;

subjecting at least a portion of said conditioned tail gas to aseparation treatment, obtaining a first product stream containing argonand a second product stream containing nitrogen.

The absorption means used during said NOx absorption stage is preferablywater.

Preferably, said conditioned tail gas is split into at least twoportions, a first portion being subjected to the aforementionedseparation treatment and a second portion being advantageously treatedin a suitable expander.

The first product stream has an argon concentration which is greaterthan the argon concentration in said conditioned tail gas, as a resultof the separation treatment. Similarly, the second product stream has anitrogen concentration which is greater than the nitrogen concentrationin said conditioned tail gas.

Preferably, said first product stream contains at least 99.5% (vol) ofargon, more preferably at least 99.95% (vol), and even more preferablyat least 99.995% (vol).

Preferably, said second product stream contains at least 99.5% (vol) ofnitrogen, more preferably at least 99.95% (vol), and even morepreferably at least 99.995% (vol).

The separation treatment is suitable to selectively separate at leastargon from nitrogen. Preferably, said separation treatment is acryogenic treatment at a temperature of no more than 133 K (−140° C.).

In a preferred embodiment, said process gas is obtained by oxidation ofa stream of ammonia in the presence of air or enriched air.Consequently, the source of nitrogen and argon contained in the tail gasis substantially the air or enriched air introduced during saidoxidation step.

Said oxidation step essentially comprises a first stage of catalyticoxidation of ammonia providing nitrogen monoxide NO and—in a smalleramount—dinitrogen monoxide N₂O, and a second stage of oxidation of NOproviding nitrogen dioxide NO₂ or dinitrogen tetroxide N₂O₄.

According to a widespread practice in the field, the compounds NO, NO₂and N₂O₄ are indicated with the general formula of NOx.

The stage of absorption of the process gas is advantageously performedin an absorption tower, wherein the NOx contained in said process gasare at least partly absorbed in said absorption means, preferably water,to provide nitric acid and the aforementioned tail gas.

Optionally, the process gas may be subjected to N₂O removal prior to theabsorption stage (so-called “secondary N₂O removal”). In someembodiments, N₂O is removed from the tail gas (so-called “tertiaryremoval”); some embodiments envisage a secondary removal and a furthertertiary removal.

The conditioned tail gas mainly contains nitrogen. Preferably said gascontains nitrogen in an amount equal to or greater than 80% (vol),preferably greater than 90% (vol), and even more preferably between 95and 98% (vol).

The conditioned tail gas also contains a non-negligible amount of argon,typically at least 0.9% (vol), preferably at least 1.0% (vol), and evenmore preferably at least 1.1% (vol).

Said conditioned tail gas may also contain small quantities of water, inan amount preferably not greater than 0.5% (vol), more preferablybetween 0.2 and 0.3% (vol).

Said conditioned tail gas preferably contains negligible amounts of NOxand N2O.

Preferably, said conditioned tail gas contains an amount of NOx notgreater than 200 ppm, more preferably not greater than 30 ppm, even morepreferably not greater than 5 ppm.

Preferably, said conditioned tail gas contains an amount of N₂O notgreater than 1000 ppm, preferably not greater than 100 ppm; morepreferably not greater than 30 ppm, even more preferably not greaterthan 10 ppm.

Given the tendency of NOx and N₂O to freeze during the separationtreatment, the presence of amounts of NOx and N₂O greater than thoseidentified above would entail a series of operational problems duringoperation of the relative plant and safety problems during stoppage ofthe plant, due to the release into atmosphere of accumulated amounts ofNOx and N₂O.

In some embodiments, the conditioned tail gas has a certain oxygencontent, preferably not greater than 5% (vol), more preferably rangingfrom 2% to 3% (vol).

The treatment of the tail gas preferably comprises a DeNOx stage bymeans of catalytic reduction, more preferably selective catalyticreduction (SCR) in the presence of a reducing agent, preferably ammonia.

In other embodiments of the invention, the treatment of the tail gascomprises a DeNOx stage of non-selective catalytic reduction (NSCR). Inthis case, the conditioned tail gas is substantially devoid of oxygenand may contain traces of hydrocarbons or hydrogen, CO, CO₂ and ammonia.

Preferably, the conditioned tail gas has a pressure greater than 4 bar,preferably ranging from 4 to 15 bar. Said pressure corresponds to thepreferred pressure for the treatment of the tail gas.

The conditioned tail gas contains no or very little carbon dioxide. Forexample the conditioned tail gas contains no more than 800 ppm of CO₂,preferably no more than 700 ppm and more preferably no more than 600ppm. The symbol ppm denotes parts per million in volume.

Said separation treatment preferably comprises a fractional distillationmaking use of the different boiling points, which are −186° C. forargon, −196° C. for nitrogen and −183° C. for oxygen (at standardconditions STP).

Preferably, said separation treatment comprises: cooling andsubsequently expanding the conditioned tail gas, obtaining a partialliquefaction, and subjecting the liquefied fraction to fractionaldistillation. Therefore the process preferably includes the fractionaldistillation of at least one of argon, nitrogen and oxygen at therespective boiling point.

The process may comprise a step of removing CO₂ before the cryogenictreatment to avoid freezing and accumulation of CO₂ in the cold box.Said removal of CO₂ preferably includes passing the gas through amolecular sieve.

Another aspect of the invention is a process for the production of astream containing argon and a stream containing nitrogen by means ofseparation treatment of a conditioned tail gas of a plant for thesynthesis of nitric acid, said conditioned tail gas being obtainedthrough the following steps:

subjecting a process gas containing NOx to a NOx absorption stage in asuitable absorption means, obtaining nitric acid and a tail gascontaining nitrogen, argon and residual NOx;

subjecting said tail gas to a treatment comprising at least one NOxremoval stage, obtaining said conditioned tail gas.

A further aspect of the invention relates to a plant for argon andnitrogen production according to the claims.

The conditioned tail gas has a greater content of argon and a muchsmaller content of oxygen compared to air. For these reasons, obtainingargon from said conditioned tail gas is substantially easier and moreprofitable than separation from air. In particular, the smaller oxygencontent (or the oxygen absence in the case of NSCR) facilitates theproduction of a product stream containing argon, because the oxygen,owing to the close boiling point, is the most difficult component toseparate from the argon.

Another advantage of said tail gas is provided by its small content ofpollutants (in particular NOx and N₂O), which allows to obtain streamsof argon and nitrogen with high purity and allows a correct operation ofthe plant, avoiding problems associated with the management of N₂O andtoxic gases such as NOx.

A further advantage is the availability of the conditioned tail gas athigh pressure (typically greater than 4 bar, for example 4-15 bar) whichallows partial liquefaction of the gas by means of expansion. In thisway the use of a dedicated compressor for said tail gas is no longernecessary, which represents an advantage from an economic point of viewcompared to a conventional air fractionation plant, since the compressorconstitutes the most costly component thereof.

Given the absence of the compressor due to the fact that the tail gas isunder pressure and given the simplified configuration of thefractionation columns due to the reduced oxygen content in the tail gas,the capital cost of the fractionation plant according to the inventionis much less than that of a conventional air fractionation plant.Consequently, argon and nitrogen may be obtained at competitive prices.

This is also true considering that the nitric acid plant must purchasefrom the market the electricity needed to compensate for the powerproduction which is lost due to the fact that the tail gas is not fullyexpanded in the expander, but is subjected to fractionation.

For these reasons, the conditioned tail gas represents a particularlyadvantageous source for the production of argon and nitrogen.

Moreover, the invention adds value to the tail gas emitted by a plantfor the nitric acid production, which in the prior art is insteaddischarged into atmosphere. In this way the invention adds a significantsource of income to the nitric acid plants. Therefore, one aspect of theinvention is represented by a combined production of nitric acid, argonand nitrogen. The nitrogen thus obtained may, for example, be sold onthe market or used to increase the production capacity of a possibleammonia plant combined with the nitric acid plant.

A further advantage of the invention is the saving of natural resourcesand of energy compared to the prior art methods of argon and nitrogenproduction which use air as raw material (i.e. distillation or selectiveadsorption), these methods requiring a significant amount of energy.

The invention is especially attractive in the case where the local argonand nitrogen market (where the nitric acid plant is present) is notbalanced with respect to the composition of the air.

The advantages of the invention will become even more clear withreference to the detailed description below, which relates to apreferred embodiment of the invention.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows a simplified diagram of the plant according to theinvention.

FIG. 2 shows a diagram of a plant for the combined production of nitricacid, argon and nitrogen according to a preferred embodiment of theinvention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The plant according to FIG. 1 comprises essentially an absorption tower4, a treatment unit 6 for the tail gas leaving said tower, an expander 7and a section 2 for the separation of a stream of argon and a stream ofnitrogen.

Said plant operates as follows.

A process gas 22 containing NOx and, in a smaller amount, N₂O and astream of water 23 are fed to an absorption tower 4. Inside said tower4, the NOx are partly absorbed in water to give a stream 24 containingnitric acid and a tail gas 25 mainly containing nitrogen and smalleramounts of oxygen, argon, water, N₂O and residual NOx.

Said tail gas 25 is sent to the treatment unit 6, where the NOx andoptionally also the N₂O are at least partly removed, providing aconditioned tail gas 26. The gas 26 leaving said treatment unit 6 haspreferably a pressure ranging from 4 to 15 bar.

Said conditioned gas 26 is advantageously split into two portions: afirst portion 26 a is expanded inside the expander 7 and discharged intoatmosphere as stream 27, and a second portion 26 b is fed to the section2 and subjected to a separation treatment, obtaining a stream 40containing argon and a stream 37 containing nitrogen.

FIG. 2 shows in greater detail the plant of FIG. 1. It comprises inparticular a section 1 for the synthesis of nitric acid and a section 2for argon and nitrogen production.

The section 1 essentially comprises a reactor 3 for catalytic oxidationof ammonia, an absorption tower 4, a heat exchanger 5, a unit 6 for NOxremoval and optionally N₂O removal, and an expander 7. Especially in thecase of high-capacity plants, said section 1 also comprises a compressorbetween the reactor 3 and the absorption tower 4.

Said section 1 operates as follows.

A stream of ammonia 20 and an air flow 21 are fed the reactor 3. Insidethe reactor 3 the ammonia is catalytically oxidized to give nitrogenmonoxide NO and—in a smaller amount—dinitrogen monoxide N₂O, and atleast a portion of NO is further oxidized to give nitrogen dioxide NO₂or dinitrogen tetroxide N₂O₄, producing a gaseous stream 22.

Said gaseous stream 22 and a stream of water 23 are introduced into theabsorption tower 4, where the NOx are at least partly absorbed to givenitric acid 24.

The absorption tower 4 also provides a tail gas 25 as head product,mainly containing nitrogen and smaller quantities of oxygen, water,argon, N₂O and residual NOx.

Said tail gas 25 is pre-heated in the exchanger 5 and subsequently fedto the unit 6. According to the example shown in FIG. 2, said unit 6comprises a DeNOx section inside which the NOx are at least partiallyremoved by means of selective catalytic reduction (SCR).

The unit 6 operates at a pressure ranging from 4 to 15 bar and providesa gas 26 mainly containing nitrogen, 2-3% oxygen, 0.2-0.3% water, NOx<30ppm and N₂O<30 ppm.

Said gas 26 is split into two portions: a first portion 26 a is expandedinside the expander 7 and a second portion 26 b is exported from thesection 1 for the synthesis of nitric acid and is fed to the section 2for argon and nitrogen production.

The expander 7 produces at least part of the power required by thecompressors (not shown) of the nitric acid section 1. The expanded gas27 is discharged into atmosphere.

The section 2 for argon and nitrogen production essentially comprises aheat exchanger 8, an expander 9, a separator 10 and a distillationapparatus 11.

According to the example of FIG. 2, said apparatus 11 comprises: a firstdistillation column 12 which operates at a pressure of about 4-5 bar, asecond distillation column 13 which operates at atmospheric pressure anda third distillation column 14 which separates argon.

Said section 2 operates as follows.

The portion 26 b of the gas coming from the section 1 is mixed with arecycling stream 32 and is fed to the heat exchanger 8 where it iscooled releasing heat to the stream 31 coming from the separator 10 andobtaining a refrigerated gas 28.

The refrigerated gas 28 is then sent to the expander 9, where it ispartly liquefied. The expander 9 is represented by a valve or by aturbine depending on the embodiments.

The partially liquefied gas 29 is fed to the separator 10. The separator10 separates a liquid phase 30 and a gaseous phase 31. The liquid phase30 is sent to the distillation apparatus 11, while the gaseous phase 31is sent to the heat exchanger 8 in order to refrigerate the incoming gas26 b and is then reintroduced into the cycle as stream 32.

In greater detail, the liquid phase 30 feeds the first column 12, whichseparates gaseous nitrogen 33 from the top and a liquid fraction 34containing nitrogen, oxygen and argon from the bottom.

The liquid fraction 34 is sent to the second column 13, while thenitrogen 33 feeds a condenser 15, wherein it condenses exchanging heatwith a tail fraction 35 of the column 13.

According to the example of FIG. 1, the stream of condensed nitrogen 36leaving the condenser 15 is split into two portions: a first portion 36a is sent to the second column 13 and a second portion 36 b is sent tothe first column 12 as reflux stream.

Said second column 13 separates nitrogen 37 and oxygen 38.

A fraction 39 containing argon and oxygen is collected in anintermediate point of the second column 13 and is sent to the thirdcolumn 14 which separates substantially pure argon 40 and oxygen 41.

EXAMPLE

In a plant which produces 500 MTD (metric tons per day) of nitric acid,a process gas containing 5-6% of NOx is obtained at the inlet of theabsorption tower. At the outlet of the aforementioned tower the tail gascontains about 300-500 ppm of NOx and at the outlet of the treatmentsection (SCR) said gas contains about 0-22 ppm. Subjecting this gas to aseparation section, about 77′000 kg/h of nitrogen and about 1′300 kg/hof argon are obtained.

1-16. (canceled)
 17. A process, comprising: subjecting a process gascontaining NO_(x) to a NO_(x) absorption stage in a suitable absorptionmeans, thereby obtaining nitric acid and a tail gas containing nitrogen,argon and residual NO_(x); subjecting said tail gas to a treatmentcomprising at least one NO_(x) removal stage, thereby obtaining aconditioned tail gas; and subjecting at least a portion of saidconditioned tail gas to a separation treatment, thereby obtaining afirst product stream containing argon and a second product streamcontaining nitrogen.
 18. The process of claim 17, wherein said firstproduct stream has an argon content of at least 99.5% (vol).
 19. Theprocess of claim 18, wherein said argon content is at least 99.95%(vol).
 20. The process of claim 18, wherein said argon content is atleast 99.995% (vol).
 21. The process of claim 17, wherein said secondproduct stream has a nitrogen content of at least 99.5% (vol).
 22. Theprocess of claim 21, wherein said nitrogen content is at least 99.95%(vol).
 23. The process of claim 21, wherein said nitrogen content is atleast 99.995% (vol).
 24. The process of claim 17, wherein saidconditioned tail gas contains an amount of argon of at least 0.9% (vol).25. The process of claim 17, wherein said conditioned tail gas containsan amount of NOx not greater than 200 ppm.
 26. The process of claim 25,wherein the amount of NO_(x) is not greater than 30 ppm.
 27. The processof claim 25, wherein the amount of NO_(x) is not greater than 5 ppm. 28.The process of claim 17, wherein said conditioned tail gas contains anamount of N₂O not greater than 1000 ppm.
 29. The process of claim 28,wherein said amount of N₂O is not greater than 100 ppm.
 30. The processof claim 28, wherein said amount of N₂O is not greater than 30 ppm. 31.The process of claim 28, wherein said amount of N₂O is not greater than10 ppm.
 32. The process of claim 17, wherein said conditioned tail gascontains an amount of oxygen not greater than 5% (vol).
 33. The processof claim 32, wherein said amount of oxygen is from 2 to 3% (vol). 34.The process of claim 17, wherein said conditioned tail gas has apressure greater than 4 bar.
 35. The process of claim 34, wherein saidpressure is from 4 bar to 15 bar.
 36. The process of claim 17, whereinthe process gas containing NO_(x) is obtained by oxidation of a streamof ammonia in the presence of air or enriched air.
 37. The process ofclaim 17, wherein the conditioned tail gas contains no more than 800 ppmof CO₂.
 38. The process of claim 17, wherein the separation treatmentincludes a cryogenic treatment.
 39. The process of 38, wherein saidseparation treatment comprises: cooling and subsequently expanding theconditioned tail gas, thereby obtaining a partial liquefaction, andsubjecting the liquefied fraction to fractional distillation.
 40. Theprocess of claim 38, further comprising removing CO₂ before thecryogenic treatment.
 41. The process of claim 40, wherein removing CO₂before the cryogenic treatment is effected with a molecular sieve.
 42. Aplant, comprising: an absorption tower, fed with a process gascontaining NO_(x) and suitable to absorb NOx in a proper absorptionmeans, providing nitric acid and a tail gas containing nitrogen, argonand residual Nox; a treatment unit for said tail gas, suitable to removeNO_(x) and give a conditioned tail gas; and a separation section,suitable to separate a first product stream containing argon and asecond product stream containing nitrogen, said separation section beingfed with at least a portion of said conditioned tail gas.
 43. The plantof claim 42, wherein the separation section comprises: a heat exchangersuitable to refrigerate said at least one portion of the conditionedtail gas, obtaining a refrigerated gas; an expander for saidrefrigerated gas, obtaining a partially liquefied gas; a separator,wherein the liquefied fraction of said partially liquefied gas isseparated from the non-liquefied fraction; and a distillation apparatus,which receives said liquefied fraction and which separates said streamcontaining argon and said stream containing nitrogen.
 44. The plant ofclaim 42, further comprising a reactor for oxidation of a stream ofammonia in the presence of air or enriched air, obtaining said processgas containing NO_(x).